The uniform and rapid processing of materials using induction generated, plasma-based processes (also referred to as inductive-coupled plasma processes) is important in the fields of semiconductor device manufacturing, packaging, optics, and the like. Many plasma processes are extensively used for the depositing or reactive etching of layers during semiconductor device fabrication. However, the radio frequency (RF at about 13.56 MHz) induction plasma source is known to produce high electron density (n.sub.e &gt;10.sup.11 cm.sup.-3) plasmas, thus providing high processing rates.
One conventional apparatus described in U.S. Pat. No. 3,705,091 to Jacob, produces a high density plasma which consists of a helical coil energized by 13 MHz RF radiation. The plasma is generated inside a low pressure cylindrical vessel within the helical coil. The coil structure induces electric fields within the plasma region which drive the discharge. High RF potentials on the coil cause capacitive coupling with the vessel walls. The capacitive coupling accelerates charged particles (electrons and ions) from the plasma into the dielectric vessel walls causing process contamination due to sputtering of the dielectric vessel walls. In addition, capacitive coupling is much less efficient than inductive coupling.
M. C. Vella et al. in Development of R.F. Plasma Generators for Neutral Beams, (Journal of Vacuum Science Technology, Vol. A3(3), pp. 1218-1221 (1985)), describe an inductive-coupled plasma process having a coil immersed in a plasma that is confined by permanent magnets. This apparatus also exhibits a degree of capacitive coupling to the discharge since the coil is in contact with the plasma.
D. K. Coultras et al. in European Patent Application 0 379 828 and Ogle in U.S. Pat. No. 4,948,458, describe inductive-coupled plasma process using a spiral coil separated from the plasma by a planar dielectric called a window. Again, high potentials on the coil cause some degree of capacitive coupling, and thus contamination of the process due to sputtering of the dielectric window.
In U.S. Pat. No. 4,918,031, Flamm et al. describe a helical resonator with a coil similar to that of Jacob in which a split cylindrical ground shield is placed between the coil and the vacuum vessel such that high fields from the coil are shorted to ground. Capacitive coupling is essentially eliminated in this configuration. However, the cylindrical geometry of this device does not allow efficient use of the ions and reactive species on large area substrates such as semiconductor wafers. Additionally, the cylindrical geometry can not be scaled for use with very large area substrates such as liquid crystal displays.
What is desired is a technique for both eliminating capacitive coupling to reduce contamination, and maintaining high inductive coupling between the coil and the plasma for improved processing rates as well as a reactor geometry which is scalable to large areas.